This invention relates generally to the field of liquid ejection and specifically to a print head for ejecting liquid droplets.
There are a growing number of technological applications that require means to inexpensively and precisely deposit small, controlled volumes of liquids. One of these important technologies involves the deposition of small solder bumps on the bond pads of integrated circuits to assist in their subsequent packaging. One of the most efficient and compact packaging schemes utilizing such solder bumps is the so-called flip chip or C4 (i.e., the Controlled Collapse Chip Connection) approach. This technology eliminates the need to wire bond connections from the bond pads to a packaging lead frame, and offers more connection pads, higher speeds, smaller die sizes, and improved heat transfer. See A. J. Babiarz, xe2x80x9cKey Process Controls for Underfilling Flip Chips,xe2x80x9d pp. 77-83, Solid State Technology, April 1997, which is incorporated herein in its entirety. Although C4 technology is somewhat costly in terms of time, materials, and equipment, and although it presents certain environmental issues, the use of solder bumped integrated circuits is growing at a significant rate. See P. Jones and James Blaha, xe2x80x9cBumped Wafers: Worth Their Weight In Gold?xe2x80x9d pp. 54-57, Advanced Packaging, January 1999, which is incorporated herein in its entirety.
The importance of this technology is underscored by the fact that a large number of companies and governmental agencies have formed the xe2x80x9cMicroFab Consortiumxe2x80x9d for the purpose of exploring and developing novel methods for applying solder bumps and other materials to integrated circuits, hybrids, optical, and other devices. The literature suggests that MicroFab has successfully developed manufacturing prototypes of piezoelectrically actuated print heads for ejecting low-melting point solder balls of well-defined sizes at rates approaching a couple of kilohertz (kHz). Although piezoelectric-based printers have several attractive characteristics, they are limited by the fact that piezoelectric strengths decrease rapidly with rising temperatures and vanish at their Curie temperatures. Curie temperatures of useful ceramics are well under 300C. Thus the ability to manipulate solder viscosity and surface tension by raising temperature is limited in such print heads. Other significant disadvantages to using piezoelectric-based print heads includes their complexity and the great difficulty in mass producing them in large, inexpensive, relatively light weight arrays.
Thus, a need exists for a print head that is functional over a wider range of temperatures to eject a greater variety of solders, brazes and other materials. Preferably, such a print head would be easily produced in large quantities out of inexpensive materials such as silicon, glasses, and metals.
In view of the foregoing considerations, an embodiment of the present invention includes a disposable print head for ejecting controlled amounts of liquid (e.g., solder) at a high rate onto the surface (e.g., an integrated circuit) in close proximity to the ejecting source. The embodiment involves the use of a controlled burst of gas that is developed by rapidly heating a metallic hydride film with a laser to disassociate the hydrogen therefrom. The pressure created by the burst of hydrogen gas is used to eject an amount of a liquid (e.g., a solder) from an appropriate conduit that is fed liquid from a reservoir. The surfaces in the print head that contact the liquid are coated to assist in priming the print head for a subsequent ejection event.